Reflective liquid crystal display (LCD) has several advantages over conventional transmissive LCD including low power consumption, light weight and good outdoor readability. In the beginning, people extended the conventional transmissive twisted-nematic (TN) LCD and super-twisted-nematic (STN) LCD to reflective display by replacing one of the polarizer with a reflective polarizer. In such a simple case, the incident light passes though the polarizer four times in total, resulting in a low light efficiency. What's more, since the reflective polarizer is placed outside the glass substrate, a serious parallax problem occurs when viewing from an oblique angle, which greatly affects the display quality, especially for high resolution reflective LCDs.
In order to solve the above mentioned parallax and low light efficiency issues, many reflective LCDs with a single polarizer have been proposed. The basic configuration of these reflective LCD includes a polarizer, a liquid crystal layer and a reflector. Since light is reflected back by the reflector and incident light passes through the polarizer twice, a reflective LCD with a single polarizer is equivalent to a transmissive LCD with a two-parallel-polarizer configuration, except for double passes of light in the liquid crystal layer. However, the conventional transmissive 90° TN-LCD does not work for the single-polarizer reflective LCD because the wave guiding effect leads to the same states for both voltage-on and voltage-off states.
In principle, the phase retardation (dΔn) of a single-polarizer reflective LCD should be around half that of a transmissive LCD because of the double passes of light. U.S. Pat. No. 5,933,207 issued to Wu on Aug. 3, 1999, which is incorporated by reference, discloses a mixed mode twisted-nematic (MTN) to solve the problems by using a polarizer, a quarter-wave film, a TN-LC layer and a reflector for direct-view display. It works very well with 90° MTN cell because of complete boundary compensation at voltage-on state, although the maximum light efficiency is only 88%. However, when the twist angle is less than 90°, the contrast ratio decreases because boundary compensation is incomplete.
U.S. Pat. No. 6,295,109 issued to Kubo et al. on Sep. 25, 2001 which is incorporated by reference, discloses using a λ/4-α film to compensate the residual phase at the voltage-on state, where α is the residual phase of the TN-LC cell at the voltage-on state. However, since the residual phase in the voltage-on state is not a pure birefringence effect, the λ/4-α film can not compensate the residual phase completely.
In addition to these two patents, use of reflective LCD mode with a single polarizer is disclosed in a monograph by S. T. Wu and D. K. Yang, “Reflective Liquid Crystal Displays”, Wiley, New York (2001). The critical issue existing in the above cited arts is that the incomplete boundary compensation results in low contrast ratio for reflective and transflective LCDs.